1. Field of the Invention
The present invention relates to an apparatus and method for filtering particulates of various sizes from miscellaneous process liquids, and more particularly to an apparatus and method that utilizes a filtration bed formed from super-buoyant media, which has a specific gravity much lower than that of the liquid being filtered.
2. Description of the Prior Art
A preliminary patentability and novelty search regarding the invention described herein has revealed the existence of the following United States Patents:
3,067,3583,469,0573,678,2403,709,3623,962,5574,032,3004,198,3014,383,9204,387,2864,415,4544,417,9624,608,1814,743,3824,839,4884,865,7344,883,0834,952,7674,963,2575,030,3535,122,2875,126,0425,178,7725,217,6075,227,0515,232,5865,573,6635,747,3115,770,0805,833,8675,932,0925,945,0056,015,497A careful review of the patents noted above has failed to reveal the concept, apparatus and method disclosed herein.
The need to remove particulates, whether contaminants or products, from process liquids is common to a wide range of processes. In the following description, the focus will be on the removal of particulate contaminants from water-based process liquids, such as swimming pools, aquariums, or sewage treatment effluents, from the deionized water used during electrical discharge machining (EDM), or from aqueous solutions such as the coolants used during conventional machining. However, the same filtration mechanism can be applied to the removal of contaminants from a variety of other process liquids including paints, oils, and hydraulic liquids. The mechanism can also be applied to the filtration and harvesting of particulate materials that form the product(s) of a process and are suspended in a process liquid.
Although a variety of methods have been developed to remove particulates from such process liquids, the most popular method is media filtration. In media filtration, particulate contaminants are strained from the process liquid in one of two ways, either by pumping the contaminated liquid through a unitary permeable element, or by pumping the liquid through a filter bed that is itself composed of small particles.
In permeable element filtration, the liquid is pumped through an element which has pores or channels that allow the liquid to pass through the element but prevent the passage of particulates larger than the pore/channel diameter. Permeable elements comprise a variety of materials, including fabric, paper, ceramic, metal, and plastic. These elements filter the liquid primarily by capturing the contaminant particles on the surface of the element, thus building up a crust or layer of contaminants on the surface. As contaminants accumulate on the surface of the element, liquid flow through the permeable element is reduced because the crust or layer of contaminants acts as an obstruction and because an increasing number of the pores or channels become blocked. As the percentage of blocked pores/channels increases and the crust or layer of contaminants becomes thicker, the pressure required to maintain a specific rate of flow of liquid through the element increases. Eventually, the pressure required exceeds the capability of the pump, or some other system component, and the contaminated element must be replaced with a new element in order to maintain the desired performance of the filtration system.
Alternatively, an attempt may be made to clean the filter element (e.g., by backwashing it with clean liquid or air) to remove the contaminants accumulated on the surface. However, even when the contaminant accumulation on the surface of such an element is removed, there are usually contaminant particles that remain lodged in the permeable element that cleaning is not totally successful in removing. Ultimately, the element must either be replaced with a new element or cleaned in a more rigorous fashion, i.e. by immersion in an acid or base solution to dissolve the contaminants. The more frequently such element replacement or stringent cleaning must be performed, the more costly this filtration process becomes.
In contrast, the second type of media filtration, namely bed filtration, uses a filter bed composed of small particles such as sand or diatomaceous earth, and is one of the most common conventional methods of removing particulate contaminants from liquids. The sand filter uses sand particles that are about 0.35 mm in diameter and fairly uniform in size. Diatomaceous earth filters use a siliceous material formed from the skeletons of small (about 100 microns [μm] in diameter) marine algal cells called diatoms. Both sand and diatomaceous earth filters use media that are substantially heavier than the process liquid being filtered, so that the media sink to the bottom of the filtration vessel forming a bed of filter media. This bed may range from several inches to several feet in thickness. Nominally, in a conventional bed filter, the process liquid is pumped, or allowed to flow (via gravity), downward through this filter bed. As the particulate-laden liquid passes through the bed, the particulates are strained from the liquid and the cleaned liquid exits at the bottom of the bed.
The bed removes the particulate contaminants via one of two processes. First, the larger particulates, which are unable to pass through the spaces between the bed grains, are trapped at the top surface of the bed. This straining effect produces a layer or crust (also called a cake) of large contaminant particles, which builds up on the surface of the bed, a mechanism called surface filtration. This cake can actually enhance the performance of the bed by helping to capture more contaminant particulates, which are retained in the crust itself, because they cannot pass through the spaces between the contaminant particles that form the crust.
Second, smaller particulates, which are carried into the bed by the liquid flow are intercepted by the bed's grains as they follow the convoluted flow pathways taken by liquid as it passes through the bed, a mechanism called depth filtration. Although smaller particulates are captured in the bed material, the smallest particulates may not be captured, as they can continue to flow through the bed and exit with the semi-cleaned liquid at the bottom of the filter bed.
Ultimately, the particulates sequestered by the bed accumulate, making it more difficult for liquid to flow downward through the bed, and thus the flow rate declines. The pressure required to force liquid through the bed then increases, and presents an excellent indication of the growing need to cleanse the bed of the accumulated particulates. Cleansing is achieved by a process of backwashing or back flushing.
During backwashing, clean liquid is vigorously pumped upwards from the bottom of the particulate bed. This upflow of liquid causes the bed to expand slightly, freeing the captured particulates and washing them upwards and out of the bed. As the bed expands, the bed particles have less interference with each other and thus settle faster, matching the up-flow rate of the liquid. This effect prevents the bed particles from being washed out of the bed along with the contaminant particulates. Typical backwash conditions are 5 to 15 minutes duration with the bed volume expanded 15 to 30%.
Although sand and diatomaceous earth filters have been successfully applied to a wide variety of filtration problems, they have a number of limitations and drawbacks. One of the most serious problems involves maintaining bed homogeneity during operation. Inhomogeneities in the bed include, for example, cracks that offer regions of less flow resistance. Such cracks lead to the formation of channels in the bed, poor distribution of the liquid flow through the bed, and thus very low particulate removal. Such inhomogeneities may also allow air to be trapped in the bed, also leading to the formation of channels and poor distribution of the liquid.
In addition, the size and cleanliness of the bed particles are extremely important to the success of the filtration process; a bed composed of large particles allows significant numbers of small particulates to pass through the filter bed along with the process liquid. On the other hand, beds composed of smaller particles can quickly become clogged with small contaminant particles, thus rendering the filter bed ineffective. The bed particles can also adsorb organic compounds on which microorganisms feed. Microbes growing on these organic compounds can bind the filter particles together and clog the bed, thus decreasing its effectiveness and shortening the interval until cleaning.
To maintain cleanliness, large volumes of clean liquid are required to backwash and clean conventional filter beds, leading to large volumes of contaminated liquid which must be treated or properly disposed. Even though backwashing is fairly effective for removing many of the particulates captured by the filter, some particulates may adhere so strongly to the bed particles that they are virtually impossible to remove. These residual contaminants reduce the effectiveness of the filter and significantly impair filter performance. Additionally, the specific gravity of the contaminant particulates is often equal to or greater than the specific gravity of the particles that make up the filter bed. In such circumstances, it is impossible to separate the heavy contaminant particles from the bed particles through a backwash process, and backwashing is therefore not effective as a cleaning method.
Thus, one of the most crucial problems with these systems, which is common knowledge to practitioners of this art, is the ineffectiveness of backwash systems for cleaning the filter media (i.e., Amirtharajah, 1978). As a consequence, in many situations, the contaminated bed cannot be cleaned and instead must be replaced with new bed material. In fact, during normal operation, both sand and diatomaceous earth filters require that the media be discarded after a certain level of media contamination has been reached. In applications that involve heavy particulate contaminant loads in the process liquid, these media may have to replaced on a daily or weekly basis, which is not economical.
An alternative method of bed filtration uses a filter bed composed of buoyant filter media particles. In this method, the media form a bed in which the majority of the media floats just beneath the surface of the process liquid. The process liquid is pumped into the bottom of the filter chamber and flows vertically upward through the bed. As the process liquid passes through the bed, contaminants are filtered from the liquid via the surface and depth filtration mechanisms described above.
Prior applications of this buoyant media method to the filtration of water (e.g., Banks, U.S. Pat. No. 4,885,083, Hsiung, et al., U.S. Pat. No. 4,608,181) have described the use of a filter media with a specific gravity of 0.7 to 0.8 or greater. For example, in Hsiung et al., the media is exactly defined as having a specific gravity of no lower than 0.8 and “most preferably” no lower than 0.9. Banks precisely specifies the specific gravity of buoyant media as 0.75 to 0.9, and “substantially equal to 0.90 to 1.0”.
Nominally, the buoyant media particles used in this type of application are also of a larger diameter than the media particles used in either sand or diatomaceous earth filters. For example, Hsiung, et al. specify the particle diameter as being preferably in the range of 1.5 to 20 mm, in contrast to the sizes of sand particles (about 0.35 mm in diameter) and diatomaceous earth (about 100 microns [μm] in diameter). Due to the relatively large size of the media particles, these buoyant media filter beds are not optimized to remove small particulate contaminants. In general, they are designed to perform larger particulate contaminant removal and some degree of biofiltration of the process liquid by the bacterial biofilm adhering to the media particles.
This buoyant media filter system, as described in the Hsiung et al. patent, actually achieves optimal operation with the media in a partially clean state. In fact, Hsiung et al. write “ . . . it is advantageous to leave a certain amount of deposited solids in a buoyant media filter, as the solids reduce the size of the pores of the filter and assist in filtration”. This requirement is often referred to as “ripening” the filter, and it means that a significant portion of the filtration capability achieved by Hsiung et al. is provided by the contaminant particles that were previously filtered and retained by the media or the microbial biofilm covering the media.
The requirement to use a “ripened” filter media bed dictates that the performance and operation of the media bed cannot be accurately characterized or predicted, as both depend on the amount and nature of the contaminant material(s) previously deposited on the buoyant media particles during the ripening process. This lack of predictable operation makes it very difficult or impossible to develop an optimal design for this type of filter.
In addition, backwashing must be performed in a gentle fashion to preserve the “ripened” layer on the filter media. If the backwash is especially vigorous, particles that were adhering to the buoyant media will be removed from the media and a portion of the buoyant media's filtration capacity would thus be lost. That capacity cannot be regained until the filter has “ripened” by again filtering a sufficient amount of contaminant particles and retaining them in the filter media in order to replenish the loss.
Thus, backwashing is typically performed by gently agitating the bed with air bubbles introduced beneath the bed and allowed to flow upwards through the bed or by gentle streams of water directed into the bed to agitate and dislodge some of the adhering contaminant particles. Accompanied by normal or reduced flow of process liquid through the buoyant media bed, these backwash methods flush only a portion of the retained contaminants from the filter bed.
The backwash system described by Hsiung et al. is the type that uses air injection and the normal flow of raw process liquid to wash excess particulates out of the media. Because the buoyant media particles have a specific gravity close to that of water, it is easy for these gentle agitating mechanisms to move the mostly submerged media around in the process liquid, and thus dislodge some of the contaminant particulates adhering to the media. Consequently, these mechanisms provide the required minimal degree of cleaning of the filter media bed. Using this method of backwashing, the amount of solids flushed from the buoyant media depends on total wash volume. However, because the media particles have a specific gravity close to that of water, they are moved easily by the backwash mechanism, and cannot be thoroughly cleaned.
Unfortunately the problems encountered in using small diameter non-buoyant media, such as sand or diatomaceous earth, are exacerbated when using small diameter buoyant media. Due to the high surface area of the small diameter media, contaminant particles that fill the interstices between the media particles can act like a glue which makes the media particles adhere to one another and form clumps which lead to the formation of non-homogeneities within the bed (just like the problem that occurs in small diameter non-buoyant media). Because the backwash systems must be relatively gentle in nature for the filter to retain its “ripened” state, these non-homogeneities cannot be removed from the bed, and the bed performance declines. This problem dictates that small diameter filter media not be used in buoyant media applications, because the ripening process itself severely limits the efficacy of the filter bed.
In addition, for cost savings, many of these buoyant media filter systems do not employ a separate backwash pump or backwash water storage system. As a result, raw process liquid is used to backwash the bed media. In these designs, maximum cleanliness of the media particles cannot be achieved because a separate, isolated pump and process liquid storage system are not utilized to provide a source of clean process liquid for backwashing. Thus, although such buoyant media filters have desirable characteristics for specific filtration applications, they do not overcome the above-stated disadvantages of conventional media bed filters.
In view of the above disadvantages with conventional apparatuses and methods, it is the principal object of the present invention to overcome the above-discussed disadvantages associated with prior media liquid filtration systems.
Another object of the present invention is to provide a liquid filtration apparatus and method that embodies a filtration bed formed from super-buoyant particles having a specific gravity less than one half that of the liquid being filtered.
A still further object of the invention is to provide a liquid filtration apparatus that embodies a filtration bed that floats on the liquid to be filtered.
Yet another object of the invention is to provide a new and improved filtering system for the removal of particulate contaminants from process liquids which incorporates a high-efficiency back-washable filter bed.
A still further object of the invention is the provision of a liquid filtration apparatus and method that in one aspect incorporates a pair of filtration housings connected in parallel.
Yet another object of the invention is the provision of a liquid filtration apparatus and method that in another aspect incorporates a pair of filtration housings connected in series.
The invention includes other objects and features of advantage, some of which, with the foregoing, will be apparent from the following description and drawings. It is to be understood that the invention is not limited to the embodiments illustrated and described, since it may be embodied in various forms within the scope of the appended claims.